1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring the thickness of the material used in a stamping or forming process, and more particularly to such an apparatus measuring the material thickness while contained within the stamping die.
2. Description of the Prior Art
In manufacturing stamped metal products, there are certain applications where knowing the material thickness as exactly as possible at the time of stamping is desirable. Some applications are products where the material thickness becomes a critical dimension, or where metal displacement in a coining operation is critical. A stack of laminations is one example of the first category; the space between the coined tips of the leads in a lead frame is an example of the second. There are also applications where the difference in material thickness from one side of the strip to the other can affect the end product.
The stacking of a series of laminations is often used, for example, in the manufacture of elements of electric motors, solenoids, transformers and the like. Typically, the individual laminas are blanked from continuous strip stock and then stacked and bound together to form the part being manufactured. However, due to imperfections of the strip stock, thickness variations often occur and may thus cause the part being formed from successive layers of the laminas to incur parallelism error (i.e., "leaning"). This parallelism error occurs because in stacking the laminas, the relatively thicker portions of the individual laminas are directly overlaying one another, and the relatively thinner portions of the laminas directly overlie one another. Parallelism error is depicted in prior art FIG. 1A. During the stacking of the individual laminations 10, one side 13 of the stack becomes higher than the opposite side 16 of the stack, which in turn results in a leaning or bending of the stack.
Techniques have been developed in the industry for attempting to overcome this leaning condition of the stack. For example, it is known in the industry to rotate the stack for symmetrical laminas so that the relatively thicker portion of each lamina does not directly overlie or underlie the relatively thicker portion of adjacent laminas. Typically the stack is rotated 180.degree. prior to the laying of each lamina. Of course, instead of rotating the stack, each lamina may be rotated while the position of the stack is kept constant to achieve the same result. U.S. Pat. No. 4,619,028 to Neuenschwander describes this type of solution to the problem associated with parallelism error.
Another problem well known in the industry is the inaccuracies of the thickness of the sheet material, commonly made of steel, used to make the individual laminas. Normal small variations in the thickness of the individual lamina will be added as a function of the number of laminations in the stack, to the point of requiring the increase or decrease in the number of laminations in the stack to remain as near to the target height as possible. An example of a stack of lamina generated by stacking individual lamina of varying thickness is depicted in prior art FIG. 1B. A predetermined target height 15 is programmed into the die controller which is known in the industry. Stack 6 consisting of nine (9) lamina is generated using lamina thickness of nominal thickness 14 and less than nominal thickness 12. This stack was slightly below the target height 15 since one additional lamina would exceed the target height 15. Stack 7 consisting of eight (8) lamina is generated in a similar fashion as stack 6 using nominal lamina thickness 14 and thicker lamina thickness 17. In either case, although the actual stack height may fall within an acceptable tolerance for the target height whether slightly larger or smaller than the target, such an occurrence cannot be determined until the stack is essentially completed and its height measured according to conventional techniques. Adding the capability to measure lamina thickness and adding a means to calculate the actual height of a stack being generated provides a means to generate a stack within a target range independent of the number of lamina need not achieve this target height. Since the stack height can be adjusted only by adding or removing laminations, this results in the stack of laminations having a height that will vary from a target height, despite either manually or automatically varying the number of laminations in the stack. Lamination stacks are commonly assembled either in the stamping die by interlocking the laminations together and also external to the die in machines made for this purpose.
A method to overcome both parallelism error and variation in material thickness is described in co-pending application Ser. No. 08/782,985, filed on Jan. 4, 1997 and its continuation-in-part application Ser. No. 08/852,017, filed on May 6, 1997, both of which are assigned to the present assignee and hereby incorporated by reference herein in this application. Briefly, these applications disclose the use of dimples on protuberances selectively formed into certain laminations. The dimples are preferably of a predetermined height and provide a means for compensation for the thickness variations of the strip stock material, obviating parallelism error and/or manufacturing a stack of laminations as near as possible to the desired height. A method using lamination rotation to overcome parallelism error can also be found in U.S. Pat. No. 5,377,115.
With either of these solutions for overcoming strip stock material thickness variations, the precise measurement of the material thickness is a critical factor. In the examples set forth previously, the effect of a thickness change is greatly magnified so extreme accuracy is required in the measuring method to both measure the variation as well as in devising the proper means of compensating for that variation. At the same time, the environment of a stamping press is relatively harsh, subjecting the measuring device to the adverse effects of high vibration (i.e., greater than 50 g force and/or 2500 Hertz), lubricant contamination, and physical damage.
Methods and apparatus are needed to accurately measure the strip stock, and hence individual lamina thickness, variations during the fabrication of structures by the stacking of laminations. Such methods and apparatus should be capable of being utilized in the harsh environment of a typical press.
It is therefore an object of the present invention to provide a method of accurately measuring the strip stock thickness during the metal stamping process.
It is a further object of the present invention to provide a lamination stamping die which contains the measuring device to measure material variations during the stamping process.
It is another object of the present invention to provide a material thickness measurement device integral to the die which provides an input to a die controller which automatically determines the need to compensate for thickness variations.